The present invention concerns a device for the continuous manufacture of metal ingots by progressive crystallization underneath a slag layer in a mold open at the bottom.
A large number of such devices is known in the art. They are used for the manufacture of metal ingots whose length is greater than the length of the casting mold. An example is the so-called electro-slag remelting process. Here one or more melt-off electrodes are remelted continuously by a molten slag layer whose area of contact with the slag layer constitutes a melt pool of varying depth. Melt-off electrodes, liquid slag and the crystallizing ingot are contained within a cylindrical or slightly conic liquid-cooled mold whose bottom consists of the ingot. The continuous operation can be maintained by uniform or stepwise drawing-off of the ingot in the downward direction with the mold at rest or by raising the mold with the ingot at rest. The melt-off electrodes are fed into the slag layer as they are consumed. The heat of fusion (melting heat) is produced by the current flow between the melt-off electrodes and the ingot through the slag layer.
The state of the art also includes another method where the temperature of the slag layer is maintained by permanent electrodes. The metal required for crystallizing the ingot is poured in the melted state through the slag layer. Such methods are designated as bottom slag foundry methods. Naturally, combinations of the two methods are conceivable.
Crystallization of the ingot inside the mold is caused by continuous heat withdrawl by cooling agents which are circulated within a hollow space in the mold. Other cooling locations are the ingot surface already protruding from the mold, and the area of contact of the ingot with a similarly cooled base. At the start of the crystallization process, this base also constitutes the lower terminal surface of the mold. As a result, there is an appreciable temperature gradient between the melted zone and the bottom edge of the mold. Because of the laws of physics, this results in an appreciable transverse contraction of the ingot increasing in the downward direction. Hence the ingot, viewed lengthwise, has a slightly conical shape. The increasing contraction appreciably impairs the sealing action of the ingot which, according to the above, constitutes the lower breech of the mold.
It is possible to provide limited compensation by a suitable conic shape of the mold. However, because of the rigidity of the mold possible time variations in the contraction are beyond control. Similarly, in the same melting device, different alloys must be remelted which result in different melting parameters with other consequences during the course of the process. Again, the rigidity of the device design does not permit an arbitrary adaptation to the changed melting parameters. It is also possible that through irregularities in the melting process there occur disturbances caused by the fact that appreciable quantities of slag and/or metal pass through the annular gap between mold and ingot into the space below. Among these are, for example, the local overheating of the slag, migration of the liquid-solid boundary of the melt, and cracks in the slag layer forming at the surface of the mold and on the ingot surface. Aside from the fact that this may endanger personnel and damage the device, such an occurrence will in most cases interrupt the melting process which then cannot be started again. The result is that that section of the ingot produced at high cost must be rejected or be used for inferior purposes.
A time-wise change of the temperature profile in the mold will appear very frequently with the so-called "hot topping" phase. As is well known, "hot topping" plays a considerable part in the manufacture of ingots, because only in this manner is it possible to eliminate or reduce the formation of faults in the ingot part produced last. Through "hot topping" a liquid phase will be maintained as long as possible in the center of the ingot surface during continuous, but reduced supply of liquid metal. Otherwise, head contraction cavities might form which would lead to the rejection of an appreciable part of the upper ingot end. As a result of the reduced heat inflow during this part of the process, the transverse contraction of the ingot even increases. Hence the annular gap is enlarged and the danger of a breakthrough of slag and/or molten metal increases. With larger ingot diameters there may easily appear an annular gap several centimeters wide causing considerable problems with respect to prevention of run-out of slag or molten metal. The difficulties are increased if the several electrodes used during the remelt process proper are replaced during the "hot topping" phase by a single electrode. This results in an appreciable change in the width/depth ratio of the melt pool.
Since the single "hot topping" electrode is located in the ingot axis, the boundary zones of the ingot are subject to more intensive cooling. Allowance must be made for the fact that the temperature gradient curve within the ingot surface, because of the better thermal conductivity of the metal, is much steeper than that inside the liquid slag. This is of particular significance because the slag above the ingot end is in the molten state for quite some time, and there is the danger that it may run through out the annular gap between mold and ingot.
Accordingly, it is an object of the present inventin to modify and improve a device of the initially described type in such a way that breakthroughs or flowout of liquid slag or molten metal is prevented.
Another object of the present invention is to provide an arrangement of the foregoing character which is simple in design and construction, and economical in operation.